Extending Phase-Noise Measurement Capability

Several techniques are available to lower the noise floor and extend the usable frequency range of commercial phase-noise measurement systems.

Phase-noise measurements are an important part of many microwave designs, especially when developing oscillators, phase-locked loops (PLLs), prescalers, frequency converters, and frequency synthesizers. Since a phase-noise measurement system represents a considerable investment, teams of designers or an entire facility often share a single phase-noise test system. Extending the system's performance can thus benefit an engineering team or group of teams. For test systems based on the PLL-with-reference-source measurement technique, it is possible to reduce the noise contribution of the reference source using dividers, or extend the frequency range to 110 GHz using mixers. This article offers several techniques for extending the usefulness of a phase-noise measurement system.

The technique of using a PLL with a reference source is very useful in research laboratories because it can be configured for numerous measurements and many types of devices (Fig. 1). In this setup, the phase difference of the two input signals to the phase detector are converted to a voltage at the detector's output. When the phase difference is set to quadrature or 90 deg., the output voltage is 0 V. Phase fluctuations from quadrature result in voltage fluctuations at the output of the detector. When quadrature is not maintained, an error can be introduced into the results based on the amount of the phase difference from quadrature.

A reference source provides the second input signal for the phase detector. The PLL controls the source and establishes phase quadrature at the phase detector using the DC voltage control capability of the reference source. The phase noise that is measured at the phase detector is the sum of the mean square phase fluctuations of the reference source and the DUT. If one source has much lower phase-noise characteristics, then the measurement results will reflect the higher phase noise of the other source.

Since the phase difference between the two signals will be small, resulting in a small change in output voltage from the phase detector, signal conditioning is required prior to baseband analysis. The signal conditioning protects the low-noise amplifier (LNA) from being subjected to mixer sum products and local-oscillator (LO) feedthrough signals. It also allows the measurement of low noise levels far from carrier even though noise close to the carrier may be large.

As in a receiver, the LNA improves the baseband analyzer's sensitivity to lowlevel signals. The baseband analysis hardware measures the voltage fluctuations to extract the desired magnitude and frequency information. Baseband analysis hardware can be a digitizer, Fast Fourier Transform (FFT) analyzer, or swept RF spectrum analyzer depending on the flexibility of the phasenoise measurement system. Besides controlling the hardware, a personal computer (PC) typically operates on the magnitude and frequency data to generate a phase-noise plot.

The typical input to an RF phase detector is limited to a maximum carrier frequency of 1.6 GHz. Some phasenoise systems use a separate phasedetector circuit for frequencies to 26.5 GHz. When using this microwave phase-detector input, it is necessary to connect a low-noise microwave reference source at the same frequency as the device under test (DUT). For example, the test a 20-GHz DUT, a 20-GHz signal generator or other reference source with about 10-dB lower noise than the DUT is needed. This microwave phase detector input is especially useful when making "paired" noise measurements on multiple microwave source devices from a prototype run or production batch. One device is "under test" and another device of the same type is used as the reference input. Multiple measurements of pairs with three devices allow the calculation of the phase noise of each device.

For signals above 1.6 GHz, most modern phase-noise test systems employ frequency downconverters to translate higher signals down to the range of the RF phase detector (Fig. 2). These downconverters can typically handle input signals from about 1.6 to 26.5 GHz. The downconverter contributes to the overall measurement noise, so it is critical that downconverters designed for phasenoise test systems exhibit very low noise. Without a low-noise design, the noise of the downconverter may become a limiting factor in the overall system measurement noise floor thereby limiting the range of DUTs that can be measured.

Since the reference source noise contributes to the overall phase-noise measurement as well, and RF signal generators are capable of superior phase-noise performance to higher-frequency microwave generators, it is possible to achieve superior phase-noise test system performance by using an RF signal generator in the sub-1-GHz range as the reference source. Having a phasenoise system with flexibility to use different reference sources can improve measurements within a particular range. One reference source could be used to measure close-to-carrier phase noise and another reference source may be selected for the best far-from-carrier measurements.

For some applications, it may be necessary to make noise measurements on millimeter-wave devices operating between 26.5 and 110 GHz. Harmonic mixers are typically used with the phase-noise test system for absolute phase-noise measurements of devices in these millimeter frequency bands (Fig. 3). Millimeter harmonic mixers are typically available in waveguide bands, e.g., 26.5 to 40 GHz, 33 to 50 GHz, 50 to 75 GHz, and 75 to 100 GHz. Harmonic mixers provide IF outputs to 1.3 GHz that serves as the input to the phase detector. To cover a broad frequency range of carriers, multiple mixers and multiple phase-noise measurements may be required. A power amplifier and high-power LO operating from 3 to 6 GHz are generally needed when using a harmonic mixer. The downconverter in some phase-noise systems may provide the LO along with a variablegain IF amplifier to boost the level of the IF output from the harmonic mixer.

If the noise of the DUT is about the same level as the reference source noise, it will be difficult if not impossible to determine the noise of the DUT. If a lowernoise reference source is not available, it may be necessary to use the measurement setup of (Fig. 4). Since noise is proportional to frequency, dividing the frequency of a source reduces the noise. For example, frequency division by 10 provides a 20 dB decrease in noise. When a divider is used with a signal generator, the noise floor of the measurement can be reduced from a few dB up to 20 dB depending on carrier frequency and offset.

For phase-noise measurements, the reference source should always be used at its highest output level. A high output signal on a signal generator typically provides the best noise performance. An attenuator between the generator's output port and the divider can help match the level of the signal to an amplitude that is optimal for the divider.

This divider technique can easily cover the range from 50 kHz to 5 GHz. Most dividers provide division numbers between 2 and 256 in powers of 2. The division ratio can be set manually-with switches or automatically using control lines. The frequency of the signal generator is then set so that the divider output provides the desired reference source frequency near the DUT's carrier. Dividers designed for use with phase-noise systems may have an integral LNA stage to boost the divider's output signal entering the phase detector. A single-ended output would be used in phase-noise systems or as a single low-noise clock source. However, some dividers have differential outputs used as low-noise differential clocks. When a divider and attenuator are combined with a reference signal generator, engineers can achieve new measurement levels of phase-noise and jitter performance, both in phase-noise measurement applications and for highquality digital clocks.

Since each laboratory may have one phase-noise system to share for numerous projects, a test system should provide the flexibility to operate at RF, microwave, and millimeter-wave frequencies. A phase-noise system that accommodates low-noise downconverters, mixers, and dividers helps serve all engineering teams, no matter what the frequency of interest. For example, the E5505A phase-noise system from Agilent Technologies (www.agilent.com) uses this architecture for the lowest noise and most flexible noise measurements. Technologies in the aerospace/defense industry, including radar, continue to drive lower noise requirements. In wirelesscommunication applications, higher bandwidth and faster data rates require lower noise components. To measure these lowernoise devices, a phase-noise system may require an extremely low-noise reference source. Using a divider, today's best signal generators can be used as a reference source to dramatically decrease the overall noise floor of the system. With the model 70429AK95 low-noise divider and E5505A system from Agilent, engineers can make phase-noise measurements on lower-noise components and devices in support of emerging commercial and military applications.

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